As a steplessly variable speed transmission of this type, there is conventionally known a technique proposed in U.S. Pat. No. 4,341,131 or Japanese Patent Application Laid-Open Gazette No. 54-35560. In this technique, as shown in FIG. 1, an HST (4'), in which a hydraulic pump (5) having a variable swash plate (51) is connected to a hydraulic motor (6) having a fixed swash plate (61'), is combined with an MT (3') including first and second planetary gear sets (7', 8'), first to third selectively-engageable clutches (10, 11, 12) for switching operational conditions of each of the planetary gear sets (7', 8') and so on. The proposed steplessly variable speed transmission performs speed control, under conditions that the relationship between the gear ratios of the two planetary gear sets (7', 8') is specifically set and the variable swash plate (51) is set to have a swash plate angle (maximum swash plate angle) identical with that of the fixed swash plate (61') at each time of mode switch in the case where the steplessly variable speed transmission is operated in divided three modes of operation.
As shown in FIG. 2, the HMT disclosed in the above-mentioned Gazette normally has a system designed such that the variable swash plate (51) of the hydraulic pump (5) is controlled so as to be changed between the three modes of operation according to the change gear ratio of the HMT and thereby rotation applied from a drive source such as an engine to an input shaft (1) of the HMT at constant rpm is transmitted to an output shaft (2) of the HMT at steplessly and continuously changed rpm. In this case, as shown by dash-double-dot line in FIG. 2, the variable swash plate (51) of the hydraulic pump (5) is controlled such that the swash plate angle is gradually increasingly or decreasingly changed at a constant rate of increase or decrease between each maximum swash plate angle (for example, 17 degrees and -17 degrees) position and a neutral position (a swash plate angle of zero) in each of the first, second and third modes or a total of three modes of operation in all. On the other hand, the swash plate of the hydraulic motor (6) is the fixed swash plate (61') and its swash plate angle is always set at the maximum swash plate angle as shown in dash-single-dot line in FIG. 2. When the variable swash plate (51) is inclined at the normal-side or reverse-side maximum swash plate angle or is in change gear ratio positions where the absolute value of the swash plate angle is equal to the swash plate angle of the fixed swash plate (61') (change gear ratio positions corresponding to respective rpm shown by I and III in FIG. 2), the switch from the first mode to the second mode and the switch from the second mode to the third mode are respectively carried out.
In the MT (3'), only the first clutch (10) is put into engagement in the first mode, only the second clutch (11) is put into engagement in the second mode, and only the third clutch (12) is put into engagement in the third mode. At each switch between modes of operation, the clutches (10, 11, 12) are each switched between engagement and disengagement. Further, at the switch of each of the clutches (10, 11, 12) between engagement and disengagement, it is necessary to fulfil a condition that the first and second planetary gear sets (7', 8') have the below-mentioned relationship between their gear ratios in order that each of the clutches (10, 11, 12) synchronizes at the same rpm and transmit rotation at a continuous change gear ratio between before and after the switch of the clutch. That is, the first and second planetary gear sets (7', 8') are conditioned that the following relationship EQU Y=X+1
is established where Y is a gear ratio between a sun gear (71') and a ring gear (73') of the first planetary gear set (7') and X is a gear ratio between a sun gear (81') and a ring gear (83') of the second planetary gear set (8') (See FIG. 3).